Add querying and displaying of which versions of code are deployed in each service/environment.

Each project can have zero or more "Existing Deployments". Periodically, the service will poke a configured URL for each of these deployments, extract the value from a specific header, and store that for display in both the index page and each project's page.

Since most of our projects expose their current version in a X-VCS-Version or similar, this information can be collected for them, and the index page can then provide at a glance an overview of which versions each project is at.